Adjustable array of fuel cell systems

ABSTRACT

An array of fuel cell systems are electrically couplable in series and/or parallel combinations to provide a variety of output powers, output current and/or output voltages. The fuel cell systems are “hot swappable” and redundant fuel cell systems may automatically replace faulty fuel cell systems to maintain output power, current and/or voltage, with or without switching. The configuration of fuel cell systems may be automatic and may be based on desired power, current and/or voltage, and/or based on the operating parameters of the fuel cell systems and/or power supply system.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This disclosure generally relates to fuel cell systems andelectric power plants incorporating them, and more particularly to powerplants including one or more arrays of fuel cell systems.

[0003] 2. Description of the Related Art

[0004] Electrochemical fuel cells convert fuel and oxidant toelectricity. Solid polymer electrochemical fuel cells generally employ amembrane electrode assembly (“MEA”) which includes an ion exchangemembrane or solid polymer electrolyte disposed between two electrodestypically comprising a layer of porous, electrically conductive sheetmaterial, such as carbon fiber paper or carbon cloth. The MEA contains alayer of catalyst, typically in the form of finely comminuted platinum,at each membrane electrode interface to induce the desiredelectrochemical reaction. In operation, the electrodes are electricallycoupled for conducting electrons between the electrodes through anexternal circuit. Typically, a number of MEAs are electrically coupledin series to form a fuel cell stack having a desired power output.

[0005] In typical fuel cells, the MEA is disposed between twoelectrically conductive fluid flow field plates or separator plates.Fluid flow field plates have flow passages to direct fuel and oxidant tothe electrodes, namely the anode and the cathode, respectively. Thefluid flow field plates act as current collectors, provide support forthe electrodes, provide access channels for the fuel and oxidant, andprovide channels for the removal of reaction products, such as waterformed during fuel cell operation. The fuel cell system may use thereaction products in maintaining the reaction. For example, reactionwater may be used for hydrating the ion exchange membrane and/ormaintaining the temperature of the fuel cell stack.

[0006] The stack's capability to produce current flow is a directfunction of the amount of available reactant. Increased reactant flowincreases reactant availability. The stack voltage varies inversely withrespect to the stack current in a non-linear mathematical relationship.The relationship between stack voltage and stack current at a given flowof reactant is typically represented as a polarization curve for thefuel cell stack. A set or family of polarization curves can representthe stack voltage-current relationship at a variety of reactant flowrates.

[0007] In most practical applications, it is desirable to maintain anapproximately constant voltage output from the fuel cell stack. Oneapproach is to employ a battery electrically coupled in parallel withthe fuel cell system to provide additional current when the demand ofthe load exceeds the output of the fuel cell stack and to store currentwhen the output of the fuel cell stack exceeds the demand of the load,as taught in commonly assigned pending U.S. patent application Ser. No.10/017,470 entitled “Method and Apparatus for Controlling Voltage From aFuel Cell System” (Attorney Docket No. 130109.436); Ser. No. 10/017,462entitled “Method and Apparatus for Multiple Mode Control of Voltage Froma Fuel Cell System” (Attorney Docket No. 130109.442) and Ser. No.10/017,461 entitled “Fuel Cell System Multiple Stage Voltage ControlMethod and Apparatus” (Attorney Docket No. 130109.446), all filed Dec.14, 2001.

[0008] The many different practical applications for fuel cell basedpower supplies require a large variety of different power deliverycapabilities. In most instances it is prohibitively costly andoperationally inefficient to employ a power supply capable of providingmore power than required by the application. It is also costly andinefficient to design, manufacture and maintain inventories of differentpower supplies capable of meeting the demand of each potentialapplication (e.g., 1 kW, 2 kW, 5 kW, 10 kW, etc.). Further, it isdesirable to increase the reliability of the power supply, withoutsignificantly increasing the cost. Thus, a less costly, less complexand/or more efficient approach to fuel cell based power supplies isdesirable.

BRIEF SUMMARY OF THE INVENTION

[0009] In one aspect, a power supply system comprises a power bus, afirst fuel cell system, a second fuel cell system, a first switchselectively operable to electrically couple the first fuel cell systemin series in the power bus, and a second switch selectively operable toelectrically couple the second fuel cell system in series in the powerbus. The fuel cell system may comprise a fuel cell stack and/or anelectrical power storage device such as a battery or super orultracapacitor electrically coupled in parallel with the fuel cellstack, and a regulating device such as a series pass element andregulating circuit and/or a reactant control system and control elementto control a partial pressure of a reactant flow to the fuel cell stack.

[0010] In another aspect, a power supply system comprises a power bus, aplurality of fuel cell systems electrically couplable in series to thepower bus, each of the fuel cell systems having a fuel cell stack and anelectrical power storage device electrically coupled in parallel withthe fuel cell stack, a plurality of switches selectively operable toelectrically decouple a respective one of the fuel cell systems from thepower bus, each of the switches responsive to an operating condition ofthe respective one of the fuel cell systems, and at least a firstredundancy switch selectively operable to electrically couple arespective one of the plurality of fuel cell systems to the power bus,the first redundancy switch responsive to an operating conditiondifferent from the operating condition of the respective one of theplurality of fuel cells. For example, the operating condition that thefirst redundancy switch is responsive to may be an operating conditionof at least one of the fuel cell systems other than the respective fuelcell system that the first redundancy switch is selectively operable toelectrically couple to the power bus.

[0011] In yet another aspect, a power supply system comprises at leasttwo fuel cell systems, and means for selectively electrically couplingthe at least two fuel cell systems in series.

[0012] In a further aspect, a method of operating a power supply systemhaving at least a first and a second fuel cell system, compriseselectrically coupling a first fuel cell system in series to a power busat a first time, determining an existence of a fault condition, inresponse to the fault condition, automatically electrically coupling asecond fuel cell system to the power bus in series at a second time, andautomatically electrically decoupling the first fuel cell system fromthe power bus at approximately the second time.

[0013] In yet a further aspect, a method of operating a power supplysystem having a plurality of fuel cell systems, a first number of thefuel cell systems electrically coupled in series to a power bus and asecond number of the fuel cell systems electrically couplable in seriesto the power bus, comprises determining an existence of a faultcondition in at least one of the first number of fuel cell systems, andautomatically electrically coupling at least one of the second number offuel cell systems to the power bus in series at a second time inresponse to the fault condition to maintain an electrical output of thepower bus of the power supply system.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0014] In the drawings, identical reference numbers identify similarelements or acts. The sizes and relative positions of elements in thedrawings are not necessarily drawn to scale. For example, the shapes ofvarious elements and angles are not drawn to scale, and some of theseelements are arbitrarily enlarged and positioned to improve drawinglegibility. Further, the particular shapes of the elements as drawn, arenot intended to convey any information regarding the actual shape of theparticular elements, and have been solely selected for ease ofrecognition in the drawings.

[0015]FIG. 1 is a schematic diagram of a hybrid fuel cell systempowering a load, the hybrid fuel cell system having a fuel cell stack, abattery, a series pass element, a first stage including a regulatingcircuit for controlling current flow through the series pass element anda second stage including a controller employing a voltage differenceacross the series pass element to reduce the energy dissipated by theseries pass element via control of reactant partial pressure , the fuelcell system for use with an illustrated general embodiment of theinvention.

[0016]FIG. 2 is a schematic diagram of a power supply system powering aload, the power supply system including a number of individual hybridfuel cells systems forming a one-dimensional array of fuel cell systemselectrically couplable in series to provide a desired power at a desiredvoltage and a desired current to the load.

[0017]FIG. 3 is a schematic diagram of a power supply system including anumber of fuel cell systems forming a two-dimensional array of fuel cellsystems electrically couplable in a variety of series and parallelcombinations.

[0018]FIG. 4 is a schematic diagram illustrating a number of the fuelcell systems of FIG. 3 electrically coupled in a series combination toprovide a desired output power at a first output voltage and a firstoutput current.

[0019]FIG. 5 is a schematic diagram-illustrating a number of the fuelcell systems of FIG. 3 electrically coupled in a parallel combination toprovide the desired output power at a second output voltage and a secondoutput current.

[0020]FIG. 6 is a schematic diagram illustrating a number of the fuelcell systems of FIG. 3 electrically coupled in a series and parallelcombination to provide the desired output power at a third outputvoltage and a third output current.

[0021]FIG. 7 is a flow diagram of a method of operating the power supplysystem of FIGS. 2 and 3 according to one exemplary embodiment whichcomprises replacing a faulty fuel cell system with a redundant fuel cellsystem.

[0022]FIG. 8 is a flow diagram of an optional step for inclusion in themethod of FIG. 7.

[0023]FIG. 9 is a flow diagram of an optional step for inclusion withthe method of FIG. 7.

[0024]FIG. 10 is a flow diagram showing a method of operating the powersupply system of FIGS. 2 and 3 according to an additional or alternativeexemplary embodiment including electrically coupling a number of fuelcell systems in a determined series and/or parallel combination toproduce at least one of a desired power, voltage and current output.

[0025]FIG. 11 is a schematic diagram of a hybrid fuel cell systempowering a load, the fuel cell system having a fuel cell stack, a seriespass element, a regulating circuit or controller for controlling currentflow through the series pass element, and an ultracapacitor basedcircuit as an electrical power storage device that simulates a battery(ultracapacitor battery simulator).

[0026]FIG. 12 is a schematic diagram of an alternative ultracapacitorbased circuit suitable for use in the fuel cell system of FIG. 11.

[0027]FIG. 13 is a schematic diagram of a further alternativeultracapacitor based circuit suitable for use in the fuel cell system ofFIG. 11.

[0028]FIG. 14 is an electrical schematic diagram of an ultracapacitorbased circuit comprising a string of ultracapacitors electricallycoupled in series, a linear mode charging current limiter, and a bypassdiode.

[0029]FIG. 15 is an electrical schematic diagram of the ultracapacitorbased circuit of FIG. 14 where the charging current limiter furthercomprises a pair of Darlington coupled transistors to limit power loss.

[0030]FIG. 16 is an electrical schematic diagram of the ultracapacitorbased circuit of FIG. 15 where the charging current limiter furthercomprises circuitry to cut off the charging current in the event of anover voltage condition.

[0031]FIG. 17 is an electrical schematic diagram of the ultracapacitorbased circuit of FIG. 16 where the charging current limiter furthercomprises circuitry to cut off charging current when a desired voltageis obtained across the ultracapacitors.

[0032]FIG. 18 is a flow diagram of one illustrated method of operating ahybrid fuel cell system.

[0033]FIG. 19 is a schematic diagram of a power system comprising one ormore rectifier arrays, fuel cell hybrid module arrays, ultracapacitorbattery simulator arrays, flywheel battery simulator arrays and/orrechargeable batteries.

[0034]FIG. 20 is a schematic diagram of one illustrated embodiment of afuel cell hybrid module array suitable for use with the power system ofFIG. 19.

[0035]FIG. 21 is a schematic diagram of a power supply system includinga number of fuel cell systems forming a two-dimensional array of fuelcell systems electrically coupled in series and parallel to provide atleast N+1 redundancy.

[0036]FIG. 22 is a schematic diagram of a power supply system includinga number of fuel cell systems forming a two-dimensional array of fuelcell systems electrically coupled in series and parallel to providemultiple voltage levels with at least N+1 redundancy.

[0037]FIG. 23 is a schematic diagram of a power supply system includinga number of fuel cell systems forming a two-dimensional array of fuelcell systems electrically coupled in series and parallel to providemultiple bipolar voltage levels With at least N+1 redundancy.

DETAILED DESCRIPTION OF THE INVENTION

[0038] In the following description, certain specific details are setforth in order to provide a thorough understanding of the variousembodiments of the invention. However, one skilled in the art willunderstand that the invention may be practiced without these details. Inother instances, well-known structures associated with fuel cells, fuelcell stacks, electrical power storage devices such as batteries,flywheels, and super- or ultracapacitors, reactant delivery systems,temperature control systems and fuel cell systems have not been shown ordescribed in detail to avoid unnecessarily obscuring descriptions of theembodiments of the invention. The terms supercapacitor andultracapacitor are used interchangeably throughout the description andclaims.

[0039] Unless the context requires otherwise, throughout thespecification and claims which follow, the word “comprise” andvariations thereof, such as, “comprises” and “comprising” are to beconstrued in an open, inclusive sense, that is as “including, but notlimited to.”

[0040] Fuel Cell System Overview

[0041]FIG. 1 shows a hybrid fuel cell system 10 providing power to aload 12 for use in an illustrated embodiment of the invention. The load12 typically constitutes the device to be powered by the hybrid fuelcell system 10, such as a vehicle, appliance, computer and/or associatedperipherals. While the hybrid fuel cell system 10 is not typicallyconsidered part of the load 12, portions of the hybrid fuel cell system10 such as the control electronics may constitute a portion or all ofthe load 12 in some possible embodiments.

[0042] The fuel cell system 10 comprises a fuel cell stack 14 composedof a number of individual fuel cells electrically coupled in series. Thefuel cell stack 14 receives reactants, represented by arrow 9, such ashydrogen and air via a reactant supply system 16. The reactant supplysystem 16 may comprise one or more reactant supply reservoirs or sources11, a reformer (not shown), and/or one or more control elements such asone or more compressors, pumps and/or valves 18 or other reactantregulating elements. Operation of the fuel cell stack 14 producesreactant product, represented by arrow 20, typically including water.The fuel cell system 10 may reuse some or all of the reactant products20. For example, as represented by arrow 22, some or all of the watermay be returned to the fuel cell stack 14 to humidify the hydrogen andair at the correct temperature and/or to hydrate the ion exchangemembranes (not shown) or to control the temperature of the fuel cellstack 14.

[0043] The fuel cell stack 14 can be modeled as an ideal battery havinga voltage equivalent to an open circuit voltage and a series resistanceR_(S). The value of the series resistance R_(S) is principally afunction of stack current Is, the availability of reactants, and time.The series resistance R_(S) varies in accordance with the polarizationcurves for the particular fuel cell stack 14. The series resistanceR_(S) can be adjusted by controlling the availability of reactants 9 todrop a desired voltage for any given current, thus allowing anapproximately uniform stack voltage V_(S) across a range of stackcurrents I_(S). The relationship between the reactant flow and theseries resistance R_(S)is illustrated in FIG. 1 by the broken line arrow13. However, simply decreasing the overall reactant and reactionpressures within the fuel cell system 10 may interfere with the overallsystem operation, for example interfering with the hydration of the ionexchange membrane and/or temperature control of the fuel cell stack. Toavoid these undesirable results, the fuel cell system 10 may adjust thereactant partial pressure, as explained in more detail below.

[0044] The fuel cell stack 14 produces a stack voltage V_(S) across ahigh voltage bus formed by the positive and negative voltage rails 19 a,19 b. The stack current I_(S) flows to the load 12 from the fuel cellstack 14 via the high voltage bus. As used herein, “high voltage” refersto the voltage produced by conventional fuel cell stacks 14 to powerloads 12, and is used to distinguish between other voltages employed byfuel cell system 10 for control and/or communications (e.g., 5V). Thus,high voltage and is not necessarily “high” with respect to otherelectrical systems.

[0045] The hybrid fuel cell system 10 comprises an electrical powerstorage device such as a supercapacitor and/or a battery 24 electricallycoupled in parallel with the fuel cell stack 14 across the rails 19 a,19 b of the high voltage bus to power the load 12. The open circuitvoltage of the battery 24 is selected to be similar to the full loadvoltage of the fuel cell stack 14. An internal resistance R_(B) of thebattery 24 is selected to be much lower than the internal resistance ofthe fuel cell stack 14. Thus, the battery 24 acts as a buffer, absorbingexcess current when the fuel cell stack 14 produces more current thanthe load 12 requires, and providing current to the load 12 when the fuelcell stack 14 produces less current than the load 12 requires. Thevoltage across the high voltage bus 19 a, 19 b will be the open circuitvoltage of the battery 24 minus the battery discharging currentmultiplied by the value of the internal resistance R_(B) of the battery24. The smaller the internal resistance R_(B) of the battery 24, thesmaller the variations in bus voltage.

[0046] An optional reverse current blocking diode D1 can be electricallycoupled between the fuel cell stack 14 and the battery 24 to preventcurrent from flowing from the battery 24 to the fuel cell stack 14. Adrawback of the reverse current blocking diode D1 is the associateddiode voltage drop. The fuel cell system 10 may also comprises otherdiodes, as well as fuses or other surge protection elements to preventshorting and/or surges.

[0047] Fuel Cell System Control Stages

[0048] The fuel cell system 10 comprises two control stages; a firststage employing a series pass element 32 and a regulating circuit 34 forcontrolling current flow through the series pass element 32, and asecond stage employing a controller 28 for adjusting reactant partialpressures to control the series resistance R_(S) of the fuel cell stack14. The first and second stages operate together, even simultaneously,in cooperation with the parallel coupled battery 24 to achieve efficientand continuous output voltage control while protecting the battery 24and fuel cell stack 14 from damage.

[0049] The first stage is a relatively fast reacting stage, while thesecond stage is a slower reacting stage relative to the first stage. Asdiscussed above, the battery 24 provides a very fast response to changesin load requirements, providing current to the load 12 when demand isgreater than the output of the fuel cell stack 14 and sinking excesscurrent when the output of the fuel cell stack 14 exceeds the demand ofthe load 12. By controlling the flow of current through the series passelement 32, the first stage ensures that the battery 24 is properlycharged and discharged in an efficient manner without damage. Bycontrolling the reactant partial pressures, and hence the seriesresistance R_(S), the second stage controls the efficiency of the fuelcell stack 14 operation (i.e., represented as the particularpolarization curve on which the fuel cell is operating). Thus, thesecond stage limits the amount of heat dissipated by the series passelement 32 by causing more energy to be dissipated via the fuel cellstack 14 (i.e., via less efficient operation).

[0050] Where the fuel cell stack 14 dissipates energy as heat, thisenergy is recoverable in various portions of the fuel cell system, andthus can be reused in other portions of the fuel cell system (i.e.,cogeneration). For example, the energy dissipated as heat may berecycled to the fuel cell stack 14 via an airflow, stack coolant, or viathe reactants. Additionally, or alternatively, the energy dissipated asheat may be recycled to a reformer (not shown), other portion of thefuel cell system 10, or to some external system. Additionally, limitingthe amount of energy that the series pass element 32 must dissipate, canreduce the size and associated cost of the series pass element 32 andany associated heat sinks.

[0051] The details of the first and second stages are discussed indetail below.

[0052] First Stage Overview, Series Pass Element Regulator

[0053] With continuing reference to FIG. 1, the first stage of the fuelcell system 10 comprises the series pass element 32 electrically coupledbetween the fuel cell stack 14 and the battery 24 for controlling a flowof current I_(S) from the fuel cell stack 14 to the battery 24 and theload 12. The first stage of the fuel cell system 10 also comprises theregulating circuit 34 coupled to regulate the series pass element 32based on various operating parameters of the fuel cell system 10. Theseries pass element 32 can, for example, take the form of a field effecttransistor (“FET”), having a drain and source electrically coupledbetween the fuel cell stack 14 and the battery 24 and having a gateelectrically coupled to an output of the regulating circuit 34.

[0054] The first stage of the fuel cell system 10 comprises a number ofsensors for determining the various operating parameters of the fuelcell system 10. For example, the fuel cell system 10 comprises a batterycharge current sensor 36 coupled to determine a battery current I_(B).Also for example, the fuel cell system 10 comprises a fuel cell stackcurrent sensor 38 coupled to determine the stack current I_(S). Furtherfor example, the fuel cell system 10 comprises a battery voltage sensor40 for determining a voltage V_(B) across the battery 24. Additionally,the fuel cell system 10 may comprise a battery temperature sensor 42positioned to determine the temperature of the battery 24 or ambient airproximate the battery 24. While the sensors 36-42 are illustrated asbeing discrete from the regulating circuit 34, in some embodiments oneor more of the sensors 36-42 may be integrally formed as part of theregulating circuit 34.

[0055] The first stage of the fuel cell system 10 may comprise a softstart circuit 15 for slowly pulling up the voltage during startup of thefuel cell system 10. The fuel cell system 10 may also comprise a fastoff circuit 17 for quickly shutting down to prevent damage to the fuelcell stack 14, for example if a problem occurs in the reactant supplysystem of the stack, where load must be removed quickly to preventdamage to the stack, or if a problem occurs with the second stagecontrol.

[0056] Second Stage Overview, Reactant Partial Pressure Controller

[0057] The second stage of the fuel cell system 10 comprises thecontroller 28, an actuator 30 and the reactant flow regulator such asthe valve 18. The controller 28 receives a value of a first voltage V₁from an input side of the series pass element 32 and a value of a secondvoltage V₂ from an output side of the series pass element 32. Thecontroller 28 provides a control signal to the actuator 30 based on thedifference between the first and second voltages V₁, V₂ to adjust theflow of reactant to the fuel cell stack 14 via the valve 18 or otherreactant flow regulating element.

[0058] Since the battery 24 covers any short-term mismatch between theavailable reactants and the consumed reactants, the speed at which thefuel cell reactant supply system 16 needs to react can be much slowerthan the speed of the electrical load changes. The speed at which thefuel cell reactant supply system 16 needs to react mainly effects thedepth of the charge/discharge cycles of the battery 24 and thedissipation of energy via the series pass element 32.

[0059] Power Supply System

[0060]FIG. 2 shows one embodiment of a power supply system 50 includinga one-dimensional array 52 of a fuel cells systems, collectivelyreferenced as 10, that are electrically couplable in series to positiveand negative voltage rails 56 a, 56 b, respectively, that form a powerbus 56 for supplying power to the load 12. A respective diode,collectively referenced as 58, is electrically coupled between thepositive and negative outputs of each of the fuel cell systems 10. Theillustrated power supply system 50 comprises a number M+1 fuel cellsystems, which are individually referenced as 10(1)-10(M+1), the numberin the parenthesis referring to the position of the fuel cell system 10in the array. The ellipses in FIG. 2 illustrate that the power supplysystem 50 may comprise additional fuel cell systems (not explicitlyshown) between the third fuel cell system 10(3) and the M^(th) fuel cellsystem 10(M). One or more of the fuel cell systems (e.g., 10(M+1)) mayserve as a “redundant” fuel cell system, being electrically coupled inseries on the power bus 56 as needed, for example, when one of the otherfuel cell systems 10(1)-10(M) is faulty or when the load 12 requiresadditional power or voltage.

[0061] The power supply system 50 may employ one or more fault switchessuch as a contactor or transistor 60, that can automatically disconnecta respective fuel cell system 10 in the event of a fault or failure. Forexample, the fault transistor 60 may open upon a fault or failure in thefuel cell system's 10 own operating condition or upon a fault or failurein the operating condition of the power supply system 50.

[0062] The power supply system 50 may employ one or more redundancyswitches, such as a contractor or transistor 62, that can manually orautomatically electrically couple a respective fuel cell system 10(M+1)to the power bus 56 based on a condition other than the fuel cellsystem's 10(M+1) own operating condition. For example, where anotherfuel cell system 10 is faulty, the redundancy transistor 62 may close toelectrically couple the redundant fuel cell system 10(M+1) to the powerbus 56 to maintain the power, voltage and current to the load 12. Alsofor example, where a higher output power is desired, the redundancytransistor 62 may close to electrically couple the redundant fuel cellsystem 10(M+1) to the power bus 56 to adjust the power, voltage andcurrent to the load 12.

[0063] While manual operation may be possible, the power supply system50 may comprise control logic 64 for automatically controlling theoperation of the redundancy switch (e.g., transistor 62).

[0064] The control logic 64 may receive an input from one or more of theother fuel cell systems 10(1)-10(M), the input relating to an operatingcondition of the respective fuel cell system 10(1)-10(M) (i.e., “connecton failure of Unit 1 through M”). For example, the control logic 64 mayreceive voltage, current and/or power measurements related to the fuelcell stack 14 and/or electrical power storage 24 of the fuel cell system10. Such measurements may include, but are not limited to, stack currentI_(S), stack voltage V_(S), battery current I_(B), and battery voltageV_(B), and/or temperature. Also for example, the control logic 64 mayreceive logical values relating to the operating condition of varioussystems of the fuel. cell system 10, including, but not limited to, anambient hydrogen level, an ambient oxygen level, and a reactant flow. Inthis respect, reference is made to commonly assigned U.S. applicationSer. No. 09/916,240, filed Jul. 25, 2001 and entitled “FUEL CELL SYSTEMMETHOD, APPARATUS AND SCHEDULING” (Attorney Docket No. 130109.409).

[0065] Additionally, or alternatively, the control logic 64 may receivean input from other components of the power supply system 50, such asvoltage and current sensors coupled to determine a voltage or current atvarious points on the power bus 56. For example, the control logic 64may receive a voltage reading corresponding to the voltage across thepower bus measured at a “top” of the one-dimensional array 52, allowingthe control logic 64 to indirectly detect a fault in one or more of thefuel cell systems 10 by detecting a measurement below an expectedthreshold value (i.e., “connect if V_(X)<M×24V”). The threshold fordetecting a fault condition may be predefined in the control logic 64 ormay be set by a user or operator via a user interface 66 such as analogor digital controls, or a graphical user interface on a special purposeor general purpose computer.

[0066] Additionally or alternatively, the control logic 64 may receivean input from the user or operator via the user interface 66 which maycomprise a set of user controls to set operating parameters such aspower, voltage, and or current thresholds, to set desired parameterssuch as desired power, desired voltage or desired current nominalvalues, to provide electrical configuration information, to provideswitching signals, and/or to signals to override the automatic operatingaspects of the control logic 64. The user interface 66 may be remotefrom the remainder of the power supply system 50. The control logic 64can be embodied in one or more of hardwired circuitry, firmware,micro-controller, application specific processor, programmed generalpurpose processor, and/or instructions on computer-readable media.

[0067] Where the output voltage of the fuel cell systems 10 can betightly controlled, such as under the first and/or second stageoperation discussed above, the series coupling of the fuel cell systems10 is possible. Thus any desired number of fuel cell systems 10 may beelectrically coupled in series to realize any integer multiple ofvoltage output of the individual fuel cell system 10. For example, whereeach fuel cell system 10 produces 24 volts across the rails 19 a, 19 b,three fuel cell systems 10(1)-10(3) are electrically couplable toproduce 72 volts across the power bus 56. More generally stated, anumber M of fuel cell systems 10 can be electrically coupled in seriesto produce M times the nominal fuel cell system voltage across the powerbus 56. Additionally, the series coupling renders the position of theredundant fuel cell system 10(M+1) in the one-dimensional array 52unimportant.

[0068]FIG. 3 shows a two-dimensional array 68 of fuel cell systems 10,arranged in a number M of rows and a number N of columns for poweringthe load 12 via the power bus 56. The fuel cell systems 10 areindividually referenced 10(1,1)-10(M,N), where the first number in theparenthesis refers to a row position and the second number in theparenthesis refers to a column position of the fuel cell system 10 inthe two-dimensional array 68. The ellipses in FIG. 3 illustrate that thevarious rows and columns of the two-dimensional array 68 may compriseadditional fuel cell systems (not explicitly shown). The diodes 58,fault and redundancy switches 60, 62, respectively, control logic 64,and user interface 66 have been omitted from FIG. 3 for clarity ofillustration.

[0069] Each of the fuel cell systems 10(1,1)-10(M,N) is individuallycouplable to the power bus 56 to provide a variety of desired outputpower, voltage or current. The fuel cell systems 10(1-M,1), 10(1-M,2),10(1-M,3)-10(1-M,N) in each column 1-M are electrically couplable inseries to one another. The fuel cell systems 10(1,1-N), 10(2,1-N),10(3,1-N)-10(M,1-N) in each row 1-N are electrically couplable inparallel to one another. From FIG. 3 and this description, one skilledin the art will recognize that the two-dimensional array 68 permits theseries coupling of fuel cell systems 10 to adjust an output power of thepower supply system 50 by adjusting an output voltage. One skilled inthe art will also recognize that the two-dimensional array 68 permitsthe parallel coupling of fuel cell systems 10 to adjust the output powerof the power supply system 50 by adjusting an output current. Oneskilled in the art will further recognize that the two-dimensional array68 permits the series and parallel coupling of fuel cell systems 10 toadjust the output power of the power supply system 50 by adjusting boththe output current and the output voltage. Thus, for the illustratedembodiment where each fuel cell system produces, for example, 1 kW at 24volts and 40 amps, a maximum output power of N×M kW is possible. Oneskilled in the art will further recognize that the one- andtwo-dimensional array structures discussed herein refer to electricallycoupable positions relative to one another, and do not necessary requirethat the fuel cell systems 54 be physically arranged in rows and/orcolumns.

EXAMPLE

[0070] FIGS. 4-6 illustrate three different electrical configurations ofthe fuel cell systems 10 of the two-dimensional array 68 of FIG. 3, toproduce a desired output power, for example 4 kW where each fuel cellsystem 10 is capable of providing 1 kW at 24 volts and 40 amps. Inparticular, FIG. 4 shows one illustrated example employing four of thefuel cell systems 10(1,1)-10(4,1) from the first column of thetwo-dimensional array 68 electrically coupled in series to provide 4 kWof power at 96 volts and 40 amps. FIG. 5 shows an illustrated embodimentof four of the fuel cell systems 10(1,1)-10(1,4) of a first row of thetwo-dimensional array 68 electrically coupled in parallel to provide 4kW of power at 24 volts and 160 amps. FIG. 6 shows an illustratedexample employing four fuel cell systems 10(1,1), 10(1,2), 10(2,1),10(2,2) of the two-dimensional array 68, where two pairs of seriescoupled fuel cell systems 10(1,1), 10(2,1) and 10(1,2), 10(2,2) areelectrically coupled in parallel to produce 4 kW of power at 48 voltsand 80 amps. One skilled in the art will recognize from these teachingsthat other combinations and permutations of electrical couplings of thefuel cell systems 10 of the two-dimensional array 68 are possible.

[0071] Operation

[0072]FIG. 7 shows a method 100 of operating the power supply system 50according to one exemplary illustrated embodiment, which is discussedwith reference to FIG. 2. The method 100 may be embodied in the controllogic 64, discussed above.

[0073] In step 102, the control logic 64 electrically couples a number Mof fuel cell systems 10(1)-10(M) in series on the power bus 56 byselectively operating appropriate ones of the switches 60, 62. In step104, the control logic 64 determines if there is a fault. For example,the control logic 64 may determine whether any of the parameters of oneof the fuel cell systems 10(1)-10(M) is outside of an acceptable range,or exceeds, or falls below, an acceptable threshold. As discussed abovethe control logic 64 may receive voltage, current and/or powermeasurements related to the fuel cell stack 14 and/or electrical powerstorage 24 of the fuel cell system 10. Additionally, or alternatively,the control logic 64 may receive logical values relating to theoperating condition of various systems of the fuel cell system 10.Additionally, or alternatively, the control logic 64 may receive aninput from other components of the power supply system 50, such asvoltage and current sensors coupled to determine a voltage or current atvarious points on the power bus 56. The control logic 64 can comprisecomparison circuitry such as a comparator, or instructions for comparingthe received values to defined range and/or threshold values, forexample, ensuring that the total voltage across the power bus 56 isabove a defined threshold or within a defined range. Alternatively, oradditionally, the control logic 64 can rely on a set of logical valuesreturned by the individual fuel cell systems 10(1)-10(M), such as a “1”or “0” corresponding to one or more operating conditions of therespective fuel cell system 10(1)-10(M).

[0074] If there is no fault, the method 100 returns to step 104,performing a monitoring loop. If there is a fault, the control logic 64electrically couples the redundant fuel cell system 10(M+1) in series onthe power bus 56 in step 106, for example, by sending an appropriatesignal to the corresponding redundant switch such as by applying asignal to a gate of the redundant transistor 62. The fuel cell systems10(1)-10(M+1) are “hot swappable” so the power supply system 50 does nothave to be shutdown.

[0075] In optional step 108, the control logic 64 electrically decouplesthe faulty fuel cell system, for example 10(3), from the power bus 56,for example, by sending an appropriate signal to the corresponding faultswitch such as by applying a signal to a gate of the fault transistor60. In optional step 110, a user or service technician replaces thefaulty fuel cell system 10(3) in the array 52 of the power supply system50. The replacement fuel cell system 10 may serve as a redundant fuelcell system for a possible eventual failure of another fuel cell system10.

[0076]FIG. 8 shows an optional step 112 for inclusion in the method 100.In step 112, an additional fuel cell system 10 is electrically coupledin series on the power bus 50 with one or more of the fuel cell systems10(1)-10(M). For example, where the faulty fuel cell system 10(3) hasbeen replaced, the replacement fuel cell system may be electricallycoupled in series to increase the power output of the power supplysystem 50.

[0077]FIG. 9 shows an optional step 114 for inclusion in the method 100.In step 114, an additional fuel cell system 10 is electrically coupledin parallel on the power bus 52 with one or more of the fuel cellsystems 10(1)-10(M). From this description, one skilled in the art willrecognize that the method 100 may employ any variety of series and/orparallel combinations of fuel cell systems 10.

[0078]FIG. 10 shows a method 130 of operating the power supply system 50according to an additional, or alternative, illustrated embodiment,which is discussed with reference to the two-dimensional array 68 ofFIG. 3. Thus, the power supply system 50 may employ the method 130 inaddition to, or alternatively from, the method 100.

[0079] In step 132, the control logic 64 determines at least one of adesired power, voltage and current output from the power supply system50. The desired values may be defined in the control logic 64 or thecontrol logic 64 may receive the desired value(s) from the user oroperator by way of the user interface 66. In step 134, the control logic64 determines an electrical configuration of series and/or parallelcombinations of a number of fuel cell systems 10(1,1)-10(M,N) to providethe desired power, voltage and/or current. In step 136, the controllogic 64 operates a number of the redundant switches such as atransistor 60 (FIG. 2, only one shown) to electrically couple respectiveones of fuel cell systems 10(1,1)10(M,N) into the determined electricalconfiguration.

[0080] The above description shows that any number of fuel cell systems10 are electrically couplable in series and/or parallel combinations toform a combined power supply system 50 for powering the load 12 at adesired voltage and current.

[0081] The fuel cell systems 10 can take the form of any of the fuelcell systems discussed above, for example, the fuel cell system 10illustrated in FIG. 1. As discussed above, the power supply system 50takes advantage of a matching of polarization curves between the fuelcell stacks 14 and the respective electrical power storage 24 to allowseries coupling of fuel cell systems. One approach to achieving thepolarization curve matching includes the first stage regulating schemegenerally discussed above. Another approach includes controlling apartial pressure of one or more reactant flows based on a deviation of avoltage across the electrical power storage 24 from a desired voltageacross the electrical power storage 24. A further approach includescontrolling a partial pressure of one or more reactant flows based on adeviation of an electrical storage charge from a desired electricalstorage charge. The electrical power storage charge can be determined byintegrating the flow of charge to and from the electrical power storage24. Other approaches may include phase or pulse switching regulating orcontrol schemes. Reasons for employing a series configuration includethe cost advantage, and the configuration having the highest efficiencyat the full output power point if the stack voltage equals the batteryfloat voltage at that point, e.g., efficiency can exceed 97% in a 24Vsystem with no R.F. noise problem. While the fuel cell systems 10 areillustrated having two stages, in some embodiments the power supplysystem 50 may incorporate one or more fuel cell systems 10 having onlyone of the stages, either the first or the second stage.

[0082]FIG. 11 shows another embodiment of a hybrid fuel cell system 10operable to power an external load 12. In contrast to the previouslydiscussed embodiments, the fuel cell system 10 of FIG. 11 employs anultracapacitor battery simulator circuit 200 as an electrical powerstorage device 24 (FIG. 1), the ultracapacitor battery simulator circuit200 being configured to simulate a battery.

[0083] The fuel cell system 10 may comprise one or more internal loads202, which represent the various active components of the fuel cellsystem 10, for example, processors, sensors, indicators, valves,heaters, compressors, fans, and/or actuators such as solenoids. Theseinternal loads 202 are typically referred to as the “balance of system”or “balance of plant” (BOP). The internal loads 202 are electricallycoupled to receive power from the fuel cell stack 14 via the power bus19 a, 19 b. The fuel cell system 10 may also comprise one or morecurrent sensors 204 and voltage sensors 206.

[0084] The ultracapacitor battery simulator circuit 200 comprises anumber of ultracapacitors C₁-C_(n) electrically coupled in seriesbetween the rails 19 a, 19 b of the voltage bus. A charging currentlimiter 208 is electrically coupled in series with the ultracapacitorsC₁-C_(n) to limit charging current to the ultracapacitors C₁-C_(n). Abypass diode D₂ is electrically coupled across the charging currentlimiter 208 to provide a path for discharge current which bypasses thecharging current limiter 208. A reverse charging diode D₃ prevents theultracapacitors C₁-C_(n) from charging in the reverse direction, forexample, when connected in series with other electrical power storagedevices 24 or hybrid fuel cell systems 10.

[0085] A number of surge diodes D_(S) are electrically coupled acrossrespective ones of the ultracapacitors C₁-C_(n). The surge diodes D_(S)equalize the voltage across each of the ultracapacitors C₁-C_(n) duringcharging, and thus may limit the voltage across any ultracapacitorC₁-C_(n) to the surge rating of the ultracapacitor C₁-C_(n). Forexample, typical ultracapacitors C₁-C_(n) may have a working voltage ofapproximately 2.5 volts. As illustrated, the ultracapacitors C₁-C_(n)may be connected in series to achieve higher working voltages. Thus, forexample, four surge diodes D_(S) electrically coupled across respectiveultracapacitors C₁-C_(n) may limit the voltage across the respectiveultracapacitor C₁-C_(n) to approximately 2.8 volts, which is the typicalsurge rating of the ultracapacitors C₁-C_(n).

[0086] The bypass diode D₂ is selected such that if the voltage on thecapacitor bank (i.e., the series coupled ultracapacitors) rises abovethe point where all of the ultracapacitors C₁-C_(n) have approximately2.8 volts across them, and all surge diodes D_(S) turn ON, the voltagedrop across the current limiter 208 will rise to limit the currentthrough the surge diodes D_(S) and prevent a short circuit.

[0087]FIG. 11 shows the charging current limiter 208 and bypass diode D₂positioned at one end of the string of ultracapacitors C₁-C_(n). FIG. 12shows the charging current limiter 208 and bypass diode D₂ positioned atthe other end of the string of ultracapacitors C₁-C_(n). FIG. 13 showsthe charging current limiter 208 and bypass diode D₂ positioned betweenthe ends of the string of ultracapacitors C₁-C_(n). Thus, it is apparentthat the charging current limiter 208 and bypass diode D₂ may bepositioned at either end, or anywhere in the string of ultracapacitorsC₁-C_(n).

[0088]FIG. 14 shows one embodiment of the charging current limiter 208in the form of a linear mode charging current limiter. The chargingcurrent limiter 208 comprises a charging current limiting transistor Q₁,feed back transistor Q₂. first resistor R₁, second resistor R₂ and Zenerdiode D₄. The charging current limiting transistor Q₁ comprises a pairof active terminals (e.g., collector and emitter) and a control terminal(e.g., base), the active terminals electrically coupled in series withthe ultracapacitors C₁-C_(n). The feedback transistor Q₂ comprises apair of active terminals (e.g., collector and emitter) and a controlterminal (e.g., base), the active terminals electrically coupled betweenrails 19 a, 19 b and the control terminal electrically coupled to theemitter of the charging current limiting transistor Q₁. The firstresistor R₁ is electrically coupled between the control terminal of thefeedback transistor Q₂ and one rail 19 b of the voltage bus. The secondresistor R₂ and Zener diode D₄ are electrically coupled between thecontrol terminal of the charging current limiting transistor Q₁ and theother rail 19 a of the voltage bus.

[0089] In use, the linear mode charging current limiter 208 passescharging current when the terminal voltage V₁-V₀ is above some definedthreshold voltage. When a voltage greater than the sum of the Zenervoltage of Zener diode D₄ and the voltage required to turn ON thecharging current limiting transistor Q₁ (e.g., approximately 0.7 volts)is applied to the terminals of the ultracapacitor battery simulatorcircuit 200, current will begin to flow into the control terminal of thecharging current limiting transistor Q₁. This causes current to flowinto the collector of the charging current limiting transistor Q₁, andbegins charging the bank of ultracapacitors C₁-C_(n). When the currentfrom the emitter of the charging current limiting transistor Q₁ issufficiently high to cause approximately 0.7 volts across the firstresistor R₁, the feedback transistor Q₂ begins to turn ON. This reducesthe current through the charging current limiting transistor Q₁. In thisway, the charging current of the bank of ultracapacitors C₁-C_(n) islimited to approximately 0.7 volts divided by the value of the firstresistor R₁. For example, if the first resistor R₁ is approximately0.175 ohms, then the charging current would be limited to approximately4 amps.

[0090] The circuit configuration of FIG. 14 also minimizes the currentdrawn from the ultracapacitors C₁-C_(n) when storing charge (i.e., whennot on float charge). When a voltage less than the sum of the Zenervoltage of Zener diode D₄ (e.g., approximately 24 volts) and the voltagerequired to turn ON the charging current limiting transistor Q₁ (e.g.,approximately 0.7 volts) is applied to the terminals of theultracapacitor battery simulator circuit 200, no current will be drawnfrom the ultracapacitors C₁-C_(n), which will consequently maintaintheir charge for relatively long intervals.

[0091]FIG. 15 shows another embodiment of the charging current limiter208, adding a pair of transistors, pair of resistors and diode, coupledin a Darlington circuit configuration Q₃, to the embodiment of FIG. 14.The Darlington circuit Q₃ is electrically coupled between the base ofthe charging current limiting transistor Q₁ and the Zener diode D₄,reducing the current I_(D3) flowing through the second resistor R₂ andthe Zener diode D₄. While slightly more complicated than the embodimentof FIG. 14, the configuration of FIG. 15 reduces power lost through thesecond resistor R₂ and Zener diode D₃.

[0092]FIG. 16 shows a further embodiment of the charging current limiter208, that adds over voltage circuitry that cuts off the charging currentin an over voltage situation, to the embodiment of FIG. 15. The overvoltage circuitry comprises an over voltage transistor Q₄, over voltageZener diode D₅, and over voltage resistor R₃. The emitters of the overvoltage transistor Q₄ and feed back transistor Q₂ are commonly coupled,and the collectors of the over voltage transistor Q₄ and feed backtransistor Q₂ are also commonly coupled. The over voltage Zener diode D₅and over voltage resistor R₃ are electrically coupled between the baseof the over voltage transistor Q₄ and one rail of the voltage bus.

[0093] When the terminal voltage of the bank of ultracapacitors C₁-C_(n)exceeds a sum of the Zener voltage of the Zener diode D₅ (e.g.,approximately 30 volts) and the voltage required to turn ON the overvoltage transistor Q₄ (e.g., approximately 0.7 volts), the over voltagetransistor Q₄ turns OFF both the feedback transistor Q₂ and chargingcurrent limiting transistor Q₁, thus preventing further charging currentfrom entering the ultracapacitors C₁-C_(n). Although the over voltagecutoff is not a feature inherent in batteries, it is desirable in ahybrid fuel cell system to account for the rise in voltage of the fuelcell stack 14 in no load conditions (e.g., open circuit voltage or OCV).The embodiment of FIG. 16 also has the advantage of limiting the heatproduced by the charging current limiting transistor Q₁ and consequentlythe size of any associated heat sink.

[0094]FIG. 17 shows yet a further embodiment of the charging currentlimiter 208, that adds circuitry to cut off charging current when thebank of ultracapacitors C₁-C_(n) reaches a desired voltage, to theembodiment of FIG. 16. The circuitry comprises a voltage settingtransistor Q₅, voltage setting Zener diode D₆, and voltage settingresistor R₄. Active terminals of the voltage setting transistor Q₅ areelectrically coupled between the base of the over voltage transistor Q₄and the rail of the voltage bus. The voltage setting Zener diode D₆ andvoltage setting resistor R₄ are electrically coupled between the base ofthe voltage setting transistor Q₅ and the bank of ultracapacitorsC₁-C_(n). The embodiment of FIG. 17 has the advantage of limiting theheat produced by the charging current limiting transistor Q₁ andconsequently the size of any heat sink associated. The embodiment ofFIG. 17 also saves power and improves overall system efficiency.

[0095] The embodiments of FIGS. 11-17 are compatible with, andcomplimentary to, previously discussed concepts, any may also beemployed with black start techniques discussed in commonly assigned U.S.application Ser. No. 10/388,191, filed Mar. 12, 2003 and titled “BLACKSTART METHOD AND APPARATUS FOR A FUEL CELL POWER PLANT, AND FUEL CELLPOWER PLANT WITH BLACK START CAPABILITY” (Attorney Docket No.130109.483).

[0096] In the embodiments of FIGS. 11-17, the fuel cell system 10behaves as a two-mode power supply. The output is controlled by twosettings: 1) an output current limit; and 2) an output voltage limit.When the load resistance is high enough to draw a current lower than theoutput current limit, the fuel cell system 10 acts as a constant voltagesource to set the output voltage limit. When the load resistance is lowenough to draw a current higher than the output current limit at theoutput voltage limit set point, the fuel cell system 10 acts as aconstant current source set to the output current limit. Chargingcurrent limiting is handled by the ultracapacitor battery simulatorcircuit 200, rather than via the series pass element 32 (FIG. 1) inbattery charging current limit mode discussed in reference to FIG. 1. Itwould be advantageous to incorporate the charging current limiting inother electrical power storage device circuitry, even where theelectrical power storage device comprises a battery rather than anultracapacitor, since this would prevent the in rush of current when,for example, a dead or discharged battery is plugged into a system withcharged batteries.

[0097] For a fuel cell system 10 employing a Ballard Nexa™ fuel cellstack, the output voltage limit would be set at or below the opencircuit voltage (OCV) of the fuel cell stack 14 (e.g., approximately54.8 volts), and the output current limit would be set such that thefuel cell stack current limit and the fuel cell system's thermal limitswere not exceeded. For example, if the output power limit is 1.3 kW, theoutput current limit would be approximately 23.7 amps.

[0098] The ultracapacitor battery simulator circuit 200 acts as a DC/DCconverter. The balance of plant 202 (FIG. 11) is typically run on 24 VDCderived from the output of the ultracapacitor battery simulator circuit200, rather than directly from the stack voltage.

[0099] The ultracapacitor battery simulator circuit 200 may have aninput voltage range of 55 volts (at OCV) to 25.5 volts (at full load).If the input voltage (i.e., stack voltage) falls below 25.5 volts, theultracapacitor battery simulator circuit 200 may lower its outputcurrent limit to the point where the input voltage does not go anylower. If the input current (i.e., stack current) rises to 48 amps, theultracapacitor battery simulator circuit 200 may lower its outputcurrent limit to the point where the input current would not any higher.

[0100]FIG. 18 shows a method 300 of operating a fuel cell system 10employing an ultracapacitor battery simulator circuit 200 according toone illustrated embodiment. In step 302, charging current is supplied,for example, from the fuel cell stack 14. In step 304, the chargingcurrent limiting transistor Q₁ and feedback transistor Q₂ limit chargingcurrent supplied to the ultracapacitors C₁-C_(n) below a chargingcurrent limit threshold. In step 306, the surge diodes D_(S) limit thevoltage across each of the ultracapacitors C₁-C_(n). In step 308, theover voltage transistor Q₄ stops the supply of charging current to theultracapacitors C₁-C_(n) in the event of an over voltage condition. Instep 310, the voltage setting transistor Q₅ stops the supply of chargingcurrent to the ultracapacitors C₁-C_(n) if the desired voltage acrossthe bank of ultracapacitors C₁-C_(n) has been attained. In step 312, theultracapacitors C₁-C_(n), discharge via the bypass diode D₂, bypassingthe charging current limiter 208.

[0101]FIG. 19 shows a power system 500 for supplying power to a load 12via rails 556 a, 556 b of a DC bus. The power system 500 receives powerfrom a power grid 502, typically in the form of three-phase AC power.The power system 500 comprises one or more rectifier arrays504(1)-504(n), which receive the AC power from the power grid 502 andrectify the power. The rectified power may be supplied to the load 12via the DC bus 556 a, 556 b. The array of rectifiers 504(1)-504(n)serves as a primary source of DC power to continuously power the load12, and to recharge a variety of electrical power storage devices 24.

[0102] The power system 500 includes an array of one or more fuel cellhybrid modules 510(1)-510(n). The array of fuel cell hybrid modules510(1)-510(n) provide continuous backup power to the load 12 via the DCbus 556 a, 556 b, for example, in the event of an interruption of thepower grid 502.

[0103] The power system 500 may also include an array of one or moreultracapacitor battery simulators 200(1)-200(n) that may store energyfor load bridging and providing surge (i.e., demand) power.Additionally, or alternatively, the power system 500 may include a flywheel battery simulator 506, that may store energy for load bridging andproviding surge power. The fly wheel battery simulator 506 may employcircuitry similar to that described for the ultracapacitor batterysimulator 200. Additionally, or alternatively, the power system 500 mayinclude one or more rechargeable batteries 508 that store energy forload bridging and providing surge power. These electrical power storagedevices may supply power to the load 12 via the DC bus formed by rails556 a, 556 b.

[0104]FIG. 20 shows a fuel cell hybrid module array 510, suitable foruse in the power system 500 of FIG. 19. The fuel cell hybrid modulearray 510, includes first and second fuel cell stacks 14 electricallycoupled in series and associated balance of plant 202, regulator 517(e.g., series pass element 32 and regulating circuit 34 of FIGS. 1 and11) and an ultracapacitor battery simulator array 200. The fuel cellhybrid module array 510 may also include a electrical power storagedevice 509, such as ultracapacitor battery simulators 200(1)-200(n), flywheel battery simulator 506, or rechargeable batteries 508 of FIG. 19.The ultracapacitor battery simulator arrays 200 provide dynamic responsefor the fuel cell hybrid modules, supplying and absorbing currentquickly in response to load requirements, while stacks 14 and balance ofplants 202 respond more slowly. Electrical power storage device 509provides energy for load bridging and providing surge power. Thisconfiguration may permit a smaller number of ultracapicitors to be usedin battery simulator arrays 200 than would be the case if they were alsorequired to provide load bridging and surge capacity. In someembodiments, in turn, this may result in an overall reduction in thenumber of ultracapacitors employed in the power plant.

[0105] Auxiliary devices, such as hydrogen supply solenoid valves 210(or ventilation fans or flow switches (not shown)), can be powered fromcenter bus 556 a. One or more equalizing circuits may be employed to aidin system startup by balancing the load to provide a reference. Theequalizing circuits may take the form of a string of resistors 212between bus 556 a, 556 b and 556 c. Other active or passive means ofbalancing the load on center bus 556 a may also be employed, if desired,such as an active controller that shares a load to maintain a particularvoltage level.

[0106]FIG. 21 shows a two-dimensional array 468 of fuel cell systems 10,arranged in a number M of rows and a number N of columns to form a powersystem for powering one or more loads 12 via the power bus 456 a, 456 b.The fuel cell systems 10 are individually referenced 10(1,1)-10(M,N)where the first number in the parentheses refers to a row position andthe second number in the parentheses refers to a column position of fuelsystem 10 in the two-dimensional array 468. The ellipses in FIG. 21illustrate that various rows and columns of the two-dimensional array468 may comprise additional fuel cell systems (not explicitly shown).While not illustrated, other multi-dimensional arrays of fuel cellsystems 10 are also possible, for example, three-dimensional arrays offuel cell systems 10.

[0107] The two-dimensional array 468 of FIG. 21 is similar to that ofFIG. 3, however, comprises links 490 electrically coupling the fuel cellsystems 10 forming a row (e.g., 10(3,1), 10(3,2), 10(3,3), . . .10(3,N)) for providing at least N+1 redundancy. The two-dimensionalarray 468 may omit the diodes 58, fault and redundancy switches 60, 62,and other elements of the previously discussed embodiments. The links490 provide redundancy, preventing the failure of a single fuel cellsystem 10 from eliminating an entire voltage string (column). Forexample, without the links 490, if fuel cell system 10(2,1) was to fail,then fuel cell systems 10(1,1), 10(3,1) through 10(M,1) would beunavailable. The links 490 prevent the loss of any individual fuel cellsystem 10 in a column from hindering the ability to fully supply theload 12. As discussed below, the links 490 may be tapped or may formtaps, to produce desired potentials on the rails of voltage buses.

[0108]FIG. 22 shows an embodiment of the two-dimensional array 468capable of providing multiple voltage levels with at least N+1redundancy. A second column of fuel cell systems 10(1,2), 10(2,2),10(3,2) . . . 10(M,2) can supply 40 amps at M×24 volts to a first load12 a via a voltage bus form by taps or rails 456 a, 456 b. A thirdcolumn of fuel cell systems 10(1,3), 10(2,3) can supply 40 amps at 48volts to a second load 12 b via a second voltage bus formed by taps orrails 456 b, 456 c. A third column of fuel cell systems 10(1,4) cansupply 40 amps at 24 volts to a third load 12 c via a voltage bus formedby taps or rails 456 b, 456 d. If the load requires more current,additional columns of fuel cell systems 10 can be added between therails of the corresponding voltage bus. Thus in the exemplary system,current can be increased in multiples of 40 amps by adding fuel cellsystems 10 to the array 468.

[0109] A first column of fuel cell systems 10(1,1), 10(2,1), 10(3,1) . .. 10(M,1) provides redundancy for each of the other fuel cell systems 10in the two-dimensional array 468. The number of fuel cell systems 10 inthe first column is equal to the number of fuel cell systems 10 in thelargest column of the array 469 to ensure at least N+1 redundancy. Byemploying a single column of fuel cell systems 10(1,1)-10(M,1),redundancy is provided to each of the other columns, without the need toprovide specific fuel cell systems for each column. This obtains atleast the desired N+1 redundancy with fewer fuel cell system 10 then inpreviously described embodiments.

[0110]FIG. 23 illustrates another embodiment of a two-dimensional array468 of fuel cell systems 10 suitable for supplying multiple bipolarvoltage levels with redundancy. The second column of fuel systems10(−2,2), 10(−1,2), 10(1,2), 10(2,2), 10(3,2), 10(4,2), 10(5,2) iscapable of supplying 40 amps at 120 volts to the first load 12 a via avoltage bus formed by taps or rails 456 a, 456 b. A third column of fuelcell systems 10(−2,3), 10(−1,3), 10(1,3), 10(2,3) is capable ofsupplying 40 amps at +48 volts to the second load 12 b via a voltage busformed by taps or rails 456 a, 456 c, or supplying 40 amps at −48 voltsto a third load 12C via a voltage bus formed by taps or rails 456 a, 456d. A fourth column of fuel cell systems 10(−1,4), 10(1,4) is capable ofsupply 40 amps at +24 volts to a fourth load 12 d via voltage bus formedby taps or rails 456 a, 456 e, or supplying 40 amps at −24 volts to afifth load 12 e via voltage bus formed by taps or rails 456 a, 456 f.Again, a first column of fuel cell systems 10(−2,1), 10(−1,1), 10(1,1),10(2,1), 10(3,1), 10(4,1), 10(5,1) provides at least N+1 redundancy toall the remaining fuel cell systems 10 in the array 468.

[0111] While not illustrated, the array 468 may employ one or moreequalizing circuits to aid in system startup by balancing the load toprovide a reference. The equalizing circuits may be as described inrelation to FIG. 20, above. Where the fuel cell systems 10 employultracapacitors, for example, equalizing devices for the intermediatevoltages across any number of series connected fuel cell systems 10 maybe added to improve the source impedance (stiffness) of the intermediatebuses.

[0112] The embodiment of FIG. 23 is particularly suitable for providingpower conditioning and/or power backup in telephone relatedapplications, such as telephone switching offices which typically employ24 volts for wireless communications such as Personal CommunicationsServices (PCS) and microwave repeater stations, 48 volts for traditionalcommunications via wire (Wireline), and 120 volts DC for switchingoperations and substations.

[0113] Conclusion

[0114] The disclosed embodiments provide a “building block” or“component” approach to the manufacture of power supply systems,allowing a manufacturer to produce a large variety of power supplysystems from a few, or even only one, basic type of fuel cell system 10.This approach may lower design, manufacturer and inventory costs, aswell as providing redundancy to extend the mean time between failuresfor the resulting end user product (i.e., the power system). Thisapproach may also simplify and reduce the cost of maintenance or repair.

[0115] Although specific embodiments of, and examples for, the powersupply system and method are described herein for illustrative purposes,various equivalent modifications can be made without departing from thespirit and scope of the invention, as will be recognized by thoseskilled in the relevant art. For example, the teachings provided hereincan be applied to fuel cell systems 10 including other types of fuelcell stacks 14 or fuel cell assemblies, not necessarily the polymerexchange membrane fuel cell assembly generally described above.Additionally or alternatively, the fuel cell system 10 can interconnectportions of the fuel cell stack 14 with portions of the electrical powerstorage device, such as cells of the battery, flywheel, orultracapacitor bank 24. The fuel cell system 10 can employ various otherapproaches and elements for adjusting reactant partial pressures, or mayoperate without regard to partial pressure. The various embodimentsdescribed above can be combined to provide further embodiments.

[0116] All of the U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, including but not limited to U.S.patent application Ser. No. 09/916,240, filed Jul. 25, 2001, andentitled “FUEL CELL SYSTEM METHOD, APPARATUS AND SCHEDULING” (AttorneyDocket No. 130109.409); U.S. patent application Ser. No. 10/017,470,filed Dec. 14, 2001, and entitled “METHOD AND APPARATUS FOR CONTROLLINGVOLTAGE FROM A FUEL CELL SYSTEM” (Attorney Docket No. 130109.436); U.S.patent application Ser. No. 10/017,462, filed Dec. 14, 2001, andentitled “METHOD AND APPARATUS FOR MULTIPLE MODE CONTROL OF VOLTAGE FROMA FUEL CELL SYSTEM” (Attorney Docket No. 130109.442); U.S. patentapplication Ser. No. 10/017,461, filed Dec. 14, 2001, and entitled “FUELCELL SYSTEM MULTIPLE STAGE VOLTAGE CONTROL METHOD AND APPARATUS”(Attorney Docket No. 130109.446); U.S. patent application Ser. No.10/388,191, filed Mar. 12, 2003 and entitled “BLACK START METHOD ANDAPPARATUS FOR A FUEL CELL POWER PLANT, AND FUEL CELL POWER PLANT WITHBLACK START CAPABILITY” (Attorney Docket No. 130109.483); U.S. patentapplication Ser. No. 10/______, filed May 16, 2003, using Express MailNo. EV347013124US and entitled “METHOD AND APPARATUS FOR FUEL CELLSYSTEMS” (Attorney Docket No. 130109.495); U.S. patent application Ser.No. 10/______, filed May 16, 2003, using Express Mail No. EV347013138USand entitled “ELECTRIC POWER PLANT WITH ADJUSTABLE ARRAY OF FUEL CELLSYSTEMS” (Attorney Docket No. 130109.499); and U.S. patent applicationSer. No. 10/______, filed May 16, 2003, using Express Mail No.EV347013141US and entitled “POWER SUPPLIES AND ULTRACAPACITOR BASEDBATTERY SIMULATOR” (Attorney Docket No. 130109.500), are incorporatedherein by reference in their entirety. Aspects of the invention can bemodified, if necessary, to employ systems, circuits and concepts of thevarious patents, applications and publications to provide yet furtherembodiments of the invention.

[0117] These and other changes can be made to the invention in light ofthe above-detailed description. In general, in the following claims, theterms used should not be construed to limit the invention to thespecific embodiments disclosed in the specification claimed, but shouldbe construed to include all fuel cell systems that operate in accordancewith the claims. Accordingly, the invention is not limited by thedisclosure but instead its scope is to be determined entirely by thefollowing claims.

We/I claim:
 1. A power supply system, comprising: a power bus; a firstfuel cell system comprising a first fuel cell stack, a first electricalpower storage device electrically coupled in parallel with the firstfuel cell stack, a first series pass element electrically coupled inbetween the first fuel cell system and the first electrical powerstorage device, and a first regulating circuit controllingly coupled tothe first series pass element; a second fuel cell system comprising asecond fuel cell stack, a second electrical power storage deviceelectrically coupled in parallel with the second fuel cell stack, asecond series pass element electrically coupled between the second fuelcell stack and the second electrical power storage device, and a secondregulating circuit controlling coupled to the second series passelement; a first switch selectively operable to electrically couple thefirst fuel cell system in series in the power bus; and a second switchselectively operable to electrically couple the second fuel cell systemin series in the power bus.
 2. The power supply system of claim 1wherein the first switch is one of a contactor and a transistor.
 3. Thepower supply system of claim 1, further comprising: a third fuel cellsystem; and a third switch selectively operable to electrically couplethe third fuel cell system in series in the power bus.
 4. The powersupply system of claim 1, further comprising: a third fuel cell system;and a third switch selectively operable to electrically couple the thirdfuel cell system in the power bus in parallel with at least one of thefirst and the second fuel cell systems.
 5. The power supply system ofclaim 1 wherein the first switch is selectively operable according to anoperating condition of the first fuel cell system, and furthercomprising: a first redundancy switch electrically coupled in serieswith the first switch and selectively operable therewith to electricallycouple the first fuel cell system in series to the power bus, the firstredundancy switch being selectively operable according to an operatingcondition of at least the second fuel cell system.
 6. The power supplysystem of claim 1 wherein the first switch is selectively operableaccording to an operating condition of the first fuel cell system, andfurther comprising: a first redundancy switch electrically coupled inseries with the first switch and selectively operable therewith toelectrically couple the first fuel cell system in series to the powerbus, the first redundancy switch being selectively operable according toa voltage across the power bus.
 7. The power supply system of claim 1wherein the first switch is selectively operable according to anoperating condition of the first fuel cell system, and furthercomprising: a first redundancy switch electrically coupled in serieswith the first switch and selectively operable therewith to electricallycouple the first fuel cell system in series to the power bus, the firstredundancy switch being selectively operable according to a desiredoutput of the power supply system.
 8. The power supply system of claim 1wherein the first switch is selectively operable according to anoperating condition of the first fuel cell system, and furthercomprising: a first redundancy switch electrically coupled in serieswith the first switch and selectively operable therewith to electricallycouple the first fuel cell system in series to the power bus, the firstredundancy switch being selectively operable according to at least oneof a desired nominal power output of the power supply system, a desirednominal voltage output of the power supply system, and a desired nominalcurrent output of the power supply system.
 9. A power supply system,comprising: a power bus; a first fuel cell system comprising a firstfuel cell stack, a first electrical power storage device electricallycoupled in parallel with the first fuel cell stack, a first reactantsupply system having a first control element operable to control apartial pressure of at least a first reactant to the first fuel cellstack; a second fuel cell system comprising a second fuel cell stack, asecond electrical power storage device electrically coupled in parallelwith the second fuel cell stack, a second reactant supply system havinga second control element operable to control a partial pressure of atleast a first reactant to the second fuel cell stack; a first switchselectively operable to electrically couple the first fuel cell systemin series in the power bus; and a second switch selectively operable toelectrically couple the second fuel cell system in series in the powerbus.
 10. A power supply system, comprising: a power bus; a plurality offuel cell systems electrically couplable in series to the power bus,each of the fuel cell systems having a fuel cell stack and an electricalpower storage device electrically coupled in parallel with the fuel cellstack; a plurality of switches selectively operable to electricallydecouple a respective one of the fuel cell systems from the power bus,each of the switches responsive to an operating condition of therespective one of the fuel cell systems; and at least a first redundancyswitch selectively operable to electrically couple a respective firstone of the plurality of fuel cell systems to the power bus, the firstredundancy switch responsive to an operating condition different fromthe operating condition of the respective first one of the plurality offuel cells.
 11. The power supply system of claim 10, further comprising:an additional number of redundancy switches, each of the additionalnumber of redundancy switches selectively operable to electricallycouple a respective one of the fuel cell systems to the power bus inresponse to an operating condition different from the operatingcondition of the respective one of the fuel cells.
 12. The power supplysystem of claim 10 wherein the operating condition that the firstredundancy switch is responsive to is an operating condition of at leastone of the fuel cell systems other than the respective first fuel cellsystem that the first redundancy switch is selectively operable toelectrically couple to the power bus.
 13. The power supply system ofclaim 10 wherein the operating condition that the first redundancyswitch is responsive to is a voltage across an output of at least one ofthe fuel cell systems other than the respective first fuel cell systemthat the first redundancy switch is selectively operable to electricallycouple to the power bus.
 14. The power supply system of claim 10 whereineach of the fuel cell systems further comprise a number of sensors fordetermining a set of operating parameters of the respective fuel cellsystem and a processor configured to determine an operating condition ofthe respective fuel cell system based on the determined set of operatingparameters, and wherein the operating condition that the firstredundancy switch is responsive to is the operating condition determinedby at least one of the processors.
 15. The power supply system of claim10 wherein the operating condition that the first redundancy switch isresponsive to is at least one of a power output of the power bus, avoltage across the power bus and a current output of the power bus. 16.The power supply system of claim 10 wherein the operating condition thatthe first redundancy switch is responsive to is at least one of adesired nominal power output of the power supply system, a desirednominal voltage output of the power supply system, and a desired nominalcurrent output of the power supply system.
 17. The power supply systemof claim 10, further comprising: a controller communicatingly coupled toreceive a set of operating parameters from each of the plurality of fuelcell systems, the controller configured to determine an operatingcondition of the fuel cell systems based on the received sets ofoperating parameters, and wherein the operating condition that the firstredundancy switch is responsive to is the operating condition determinedby the controller.
 18. The power supply system of claim 10, furthercomprising: a first diode electrically coupled in series in the powerbus between the first and the second fuel cell systems.
 19. The powersupply system of claim 10 wherein the electrical power storage devicesare at least one of a number of battery cells and a number ofsuper-capacitors.
 20. A power supply system, comprising: a power bus; atleast two fuel cell systems; and means for selectively electricallycoupling the at least two fuel cell systems in series in the power bus,wherein each of the fuel cell systems comprises a fuel cell stack and anelectrical power storage device electrically coupled in parallel withthe fuel cell stack, the electrical power storage device comprising atleast one of a battery and an ultracapacitor electrically coupled to acharging current limiter.
 21. The power supply system of claim 20wherein each of the fuel cell systems comprises a fuel cell stack and anelectrical power storage device electrically coupled in parallel withthe fuel cell stack, and means for controlling a flow of current fromthe fuel cell stack to the electrical power storage device and the powerbus.
 22. The power supply system of claim 20 wherein each of the fuelcell systems comprises a fuel cell stack and an electrical power storagedevice electrically coupled in parallel with the fuel cell stack, aseries pass element electrically coupled between the fuel cell stack andthe electrical power storage device, and regulating means for regulatinga series pass element electrically coupled between the fuel cell stackand the electrical power storage device.
 23. The power supply system ofclaim 20 wherein the means for selectively electrically coupling the atleast two fuel cell systems in series comprises a respective switch foreach of the fuel cell systems, each of the switches responsive to anoperating condition of at least one of the other fuel cell systems. 24.The power supply system of claim 20 wherein the means for selectivelyelectrically coupling the at least two fuel cell systems in seriescomprises a respective first switch and second switch for each of thefuel cell systems, each of the first switches responsive to an operatingcondition of at least one of the other fuel cell systems and each of therespective second switches responsive to an operating condition of therespective fuel cell system.
 25. The power supply system of claim 20wherein the means for selectively electrically coupling the at least twofuel cell systems in series comprises a controller responsive to arequest for at least one of a desired power output, a desired currentoutput and a desired voltage output of the power supply system.
 26. Amethod of operating a power supply system having at least a first and asecond fuel cell system, the method comprising: electrically coupling afirst fuel cell system in series to a power bus at a first time;determining an existence of a fault condition; in response to the faultcondition, at a second time automatically electrically coupling a secondfuel cell system to the power bus in series; and automaticallyelectrically decoupling the first fuel cell system from the power bus.27. The method of claim 26 wherein determining an existence of a faultcondition includes determining an operating condition of the first fuelcell system.
 28. The method of claim 26 wherein determining an existenceof a fault condition includes determining at least one of an outputpower, output voltage and output current of the first fuel cell system.29. The method of claim 26 wherein determining an existence of a faultcondition includes monitoring at least one of a battery current, abattery voltage, and a stack current in the first fuel cell system. 30.The method of claim 26 wherein determining an existence of a faultcondition includes monitoring at least one of an ambient hydrogen level,an ambient oxygen level, and a reactant flow in the first fuel cellsystem.
 31. The method of claim 26, further comprising: electricallycoupling a third fuel cell system to the power bus in series with thefirst fuel cell system at a third time, the third time being before thesecond time.
 32. The method of claim 26, further comprising:electrically coupling a third fuel cell system to the power bus inparallel with the first fuel cell system at a third time, the third timebeing before the second time.
 33. The method of claim 26 whereinelectrically coupling a first fuel cell system in series to a power busat a first time includes closing a first switch electrically coupledbetween the fuel cell and the power bus and wherein electricallydecoupling the first fuel cell system from the power bus includesopening a redundancy switch electrically coupled between the fuel celland the power bus.
 34. A method of operating a power supply systemincluding a plurality of fuel cell systems, a first number of the fuelcell systems electrically coupled in series to a power bus and a secondnumber of the fuel cell systems electrically couplable in series to thepower bus, comprising: determining an existence of a fault condition inat least one of the first number of fuel cell systems; and automaticallyelectrically coupling at least one of the second number of fuel cellsystems to the power bus in series at a second time in response to thedetermination of the existence of the fault condition to maintain anelectrical output on the power bus of the power supply system.
 35. Themethod of claim 34, further comprising: automatically electricallydecoupling the at least one of the first number of fuel cell systemshaving the fault condition from the power bus.
 36. The method of claim34, further comprising: for each of the fuel cell systems electricallycoupled to the power bus, automatically regulating a flow of currentfrom a fuel cell stack with respect to an electrical power storagedevice electrically coupled in parallel across the fuel cell stack byway of at least one of a respective series pass element and a respectiveregulating circuit and a reactant delivery system and a control element.37. The method of claim 34, further comprising: replacing the at leastone of the fuel cell systems having the fault condition with areplacement fuel cell system.
 38. The method of claim 34, furthercomprising: replacing the at least one of the fuel cell systems havingthe fault condition with a replacement fuel cell system; andelectrically coupling the replacement fuel cell system in series to thepower bus in response to the additional fault condition to maintain anelectrical output on the power bus of the power supply system
 39. Themethod of claim 34 wherein automatically electrically coupling at leastone of the second number of fuel cell systems to the power bus in seriesat a second time in response to the determination of the existence ofthe fault condition to maintain an electrical output on the power bus ofthe power supply system includes activating a first transistorelectrically coupled between the fuel cell and the power bus inproportion to a decrease associated with the fault condition in at leastone of an output power, an output current, and an output voltage of theat least one of the first number of fuel cell systems.
 40. A method ofoperating a power supply system having a plurality of fuel cell systems,at least a number of the fuel cell systems selectively couplable inseries on a power bus, the method comprising: determining an existenceof a fault condition in at least one of the fuel cell systemselectrically coupled in series on the power bus; and automaticallyactivating a linear control element electrically coupling a redundantone of the fuel cell systems in series to the power bus in response tothe determined existence of the fault condition to maintain anelectrical output on the power bus of the power supply system.
 41. Themethod of claim 40, further comprising: determining an existence of afault condition in at least one of the fuel cell systems electricallycoupled in series on the power bus; and automatically activating atransistor electrically coupling a redundant one of the fuel cellsystems in series to the power bus in response to the determinedexistence of the fault condition to maintain an electrical output on thepower bus of the power supply system.